Perturbation theory (PT) provides an analytic model of the density field on large scales [1]}.
Two-point correlators in perturbative models are limited by the scale at which nonlinear effects dominate the dark matter dynamics
which is a much
smaller scale than the minimum scale to which current surveys are sensitive [2]}.
However, PT models remain attractive due to the control over theoretical errors they afford in their domain of validity.
Extensions of perturbation theory based on an effective fluid description of the density field (EFT) have pushed deeper into the quasi-linear regime [3]}, [4]}, [5]}, [6]}, [7]}.
Such extended perturbation theory models have recently been used successfully for analysis of cosmological parameters, though with nuisance parameters that are fitted to numerical simulations (e.g. [8]}).
However, there is a limit to any perturbative model, even in the EFT framework, as non-perturbative effects and nonlinear gravitational evolution dictate the behavior of the density field on scales less than a few \(\rm {Mpc}/h\) .
In fact, it is clear that perturbation theory does not actually converge to the fully nonlinear result on smaller scales, at least in one dimension, at infinite order [9]}, [10]}.
This is due to the fundamentally non-perturbative nature of small-scale dark matter dynamics.
